Field of the Invention
The invention relates to a process for the surface treatment of alloys comprising nickel and iron, in particular of stainless steel, and/or of a nickel alloy comprising 10-70% nickel. The invention also relates to uses for such a process and to a steel or a nickel alloy produced by such a process.
The steels are, for example, austenitic chrome-nickel steels, and the nickel alloys are, for example, of the Incoloy 800, Inconel 600 or similar types. One application of the process can be in nuclear technology for reducing the later absorption of activity (contamination) of components of the primary circulation of water-cooled nuclear power stations before a new installation or after a decontamination.
In water-cooled nuclear power stations such as, for example, boiling-water reactors (BWR) and pressurized-water reactors (PWR), oxide layers form, owing to reaction with the hot water and/or steam, on the wetted surfaces which for the major part consist of zirconium alloys and austenitic chromium-nickel steels (so-called stainless steel). Part of these oxide layers passes, owing to dissolution or erosion, into the water circulations and can be activated in the neutron field. If the activated corrosion products are incorporated outside the reactor core on surfaces of components into oxide layers present therein or deposited thereon as particles, these components are radioactively contaminated. Components at risk of contamination are, in the pressurized-water reactor, above all main coolant pumps and steam generators, and, in the case of older boiling-water reactors with external circulation, the components at risk are the recirculation lines and the reactor water purification system.
In order, then, to avoid an impermissible radiation exposure of the operating personnel during operation, inspection, maintenance work and repairs, this contamination must be minimized as far as possible. This can be effected by a careful selection of the materials and the operating parameters such as, for example, the water chemistry. If the contamination nevertheless rises to inadmissible values, such systems must be decontaminated. This is done by a chemical treatment, by means of which the oxide layer and thus the activated corrosion products contained therein are removed.
In the past, many part-systems of nuclear power stations were routinely decontaminated, such as the main coolant pumps, the steam generators or parts thereof in pressurized-water reactors, and the circulation loops and the purification system in the case of a boiling-water reactor. These decontamination processes are nowadays state of the art and are commercially available. As a rule, decontamination factors of between 10 and far more than 100 are achieved in this way.
For economic and technical reasons, only the directly interfering contaminants are removed in most cases, whereas the predominant part of the surfaces such as, for example, the surfaces of the fuel elements, are not treated, in order to minimize the volume of the radioactive wastes which arise during the decontamination and must ultimately be dumped. When the components thus cleaned are taken back into operation, their surfaces in contact with the coolant are very rapidly covered again with an oxide layer. This oxide layer reaches an equilibrium with the activation products which are present in the coolant or which pass into the coolant from the non-decontaminated surfaces. The consequence thereof is a very rapid recontamination of the cleaned surfaces. Even when components are replaced, a very rapid contamination thereof is observed. The recontamination of cleaned surfaces or the contamination of newly installed surfaces can, in a short time, assume values which are higher than those before the decontamination. This has been observed, for example, in the circulation loops in a nuclear power station after the replacement of the circulation loops.
In the past, diverse attempts have been made to pretreat the decontaminated surfaces or the surfaces newly to be used in such a way that the contamination starts only to a reduced extent. For this purpose, the following approaches are available in principle:
Reduction in the activation products available. This can be effected by a so-called complete system decontamination including the fuel elements. A great disadvantage is that large volumes of radioactive wastes arise. PA1 The preparation of surfaces which are contaminated more slowly, for example by electropolishing. This is, however, practicable only in the replacement of systems and was not successful in the case of a nuclear power station selected for experiment. PA1 Coating of the cleaned or new surface with a non-contaminated oxide layer. This can be done by various processes such as, for example, with oxygen-containing steam or with water having high oxygen contents. This requires treatments for relatively long periods and/or at high temperatures. These processes have so far not been very successful as, for example, the treatment of the new and electropolished recirculation line in a selected nuclear power station remained without any noticeable effect. PA1 a simple treatment, as far as possible in the power station itself; PA1 short treatment time and low process temperatures; PA1 unproblematic auxiliaries, i.e. non-hazardous chemicals which cannot lead to consequential damage or long-term damage, even if residues remain in the systems; PA1 the surfaces or protective layers produced must in the subsequent operation of the plant be effective and stable for a very long time and, in particular, they must not become detached; PA1 the components must not be damaged by the treatment and PA1 produced protective layers and layers which form during the subsequent normal operation of the nuclear power stations must be removable again by decontamination processes nowadays familiar in practice.